The present invention relates to a remote monitor control system comprising a single master station and a plurality of terminal stations, or more in particular to a remote monitor control system suitable for detecting a change in the conditions of an object being monitored within a short time.
As a remote control system of this type, explanation will be made about a power line carrier system using a distribution line for signal transmission between a master station and terminal stations by application of a zero-phase carrier transmission system.
FIG. 1 is a circuit diagram showing the principle of a zero-phase carrier transmission system of such a type.
In FIG. 1, reference numeral 1 designates a transmitter, numeral 2 a sync circuit, numeral 3 a control command generator, numeral 4 a three-phase ground static capacity, numeral 5 a capacitor, numeral 6 a receiver, numeral 7 a vector combiner, numeral 8 a sync detector, and numeral 9 a three-phase capacitor voltage divider. Reference characters E.sub.ab, E.sub.bc, E.sub.ca designate power supplies, a, b, c three-phase distribution lines, and L.sub.1, L.sub.2, L.sub.3 loads. Numeral 10 designates a master station comprising a transmitter 1 and a receiver 6, which are controlled by a processor including a control processor 11, an input/output unit 12, and a memory unit 14. Numeral 20 designates a terminal station, comprising a receiver 6 and a transmitter 1, which are controlled by a processor including a processing control unit 15 and an output unit 16.
Now, this monitor system employing a zero-phase carrier transmission system will be explained with reference to FIG. 2. A monitor system of zero-phase carrier transmission type is disclosed in a reference entitled "Development and Field Test on a Distribution Line Carrier Communication System", by Akira Miyahara et al., IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 3, July 1986.
An output signal corresponding to a transmission code (FIG. 2(A)) from a control processor 11 is applied to a sync circuit 2 through a control command generator 3. With one of the three phases as a reference (such as phase 3), the frequency of the reference phase and the output signal mentioned above are applied to the sync circuit 2, and according to these signals, an output of the sync circuit 2 is formed, whereby a small-capacity capacitor 5 inserted between the phase C of the three-phase distribution line and ground is turned on and off by switching means. The turning on and off of the capacitor 5 generates an imbalance of the three-phase ground static capacitor 4 thereby to generate a zero-phase voltage (FIG. 2(B)) corresponding to the transmission code over the whole distribution system. The zero-phase voltage (FIG. 2(B)) is transmitted without any substantial attenuation to the end of the distribution system.
In the receiver 6, on the other hand, each voltage produced from the three-phase voltage divider 9 is combined by the vector combiner 7 as a zero-phase voltage output. The output of the vector combiner 7 and the output of the phase-C capacitor voltage divider are applied to the sync detector 8. The sync detector 8 performs the integrating operation, and a DC component is taken out of the output of the integrating operation as shown in FIG. 2(C), thereby reproducing the transmission code as shown in FIG. 2(D).
In a remote monitor system for data transmission comprising the master station 10 including the transmitter 1 and the receiver 6 and a plurality of terminal stations 20 for performing the monitoring of the conditions and controlling the operation of the switches, etc. installed on the distribution lines, a common concept for enabling the master station 10 to grasp the conditions of all the switches is to transmit data by a polling system. The polling system is one in which communication is established one by one with all the terminal stations under the control of the master station for data transmission relating to the conditions of switches, etc.
In the case of a polling system, however, as shown in FIG. 3, the master station 10 makes an inquiry to terminal stations 20-1, 20-2, 20-3, . . . , 20-n in sequence, and these terminal stations answer the inquiries upon receipt thereof from the master station 10. If the conditions of the switches, for example, of all the terminal stations 20-1, 20-2, 20-3, . . . , 20-n are to be grasped, a transmission time proportional to the number of the terminal stations 20 is required, which poses the problem of a long time being required to grasp a change in the conditions of a given switch that may occur.
Now, calculation will be made of the time required for communication with the number n of terminal stations 20 under the above-described polling system.
The communication time t with given terminal station 20 is the sum of the transmission time from the master station 10 to the terminal station 20 (FIG. 3(t.sub.A)) and the response time from the particular terminal station 20 to the master station (FIG. 3(t.sub.B)). Therefore, the total time T required for completing the communications with the number n of terminal stations is given as EQU T=n.times.t (1)
It is understood from this that the communication time proportional to the number of terminal stations is required.
FIG. 4 shows an example of the transmission format.
Apart from this communication system, the communication according to a power line carrier system with a distribution line as a signal transmission path requires a technique described below to improve the transmission reliability.
(I) Double-transmission system PA0 (II) nCr code system PA0 (III) Parity check system
In this system, a "word" having the same content is transmitted twice to improve the reliability (sometimes with an inverted code for the second word.) PA1 This system is such that the number of codes "1" and codes "0" making up each word is predetermined. PA1 In this system, the number of codes "1" making up a word is predetermined as either an even or an odd number.
Various ideas as mentioned above are required if the transmission reliability is to be improved. As a result, the amount of transmission necessary for communication increases to a considerable degree (Three terminal station addresses are required as shown in FIGS. 4A and 4B, for instance.).
Assume that each word comprises eight codes and the above-mentioned transmission system is used. The amount of transmission in the transmission format shown in FIG. 4 is such that signals of the following format (a total of 72 bits) is generated from the master station to the terminal stations:
______________________________________ Sync signal Eight codes Command codes 8 bits .times. 2 (double transmission) = 16 codes Addresses (8 .times. 2 (double transmission)) .times. 3 (digits) = 48 codes ______________________________________
Signals of the following format (a total of 72 bits) are transmitted from the terminal stations to the master station.
______________________________________ Sync signal Eight codes Switch conditions 8 bits .times. 2 (double transmission) = 16 codes Addresses (8 .times. 2) .times. 3 = 48 codes ______________________________________
It is thus seen that a total of 144 bits including 72 bits of transmission codes from the master station and 72 bits of response codes are required for the communication with each terminal station. If the system frequency is the transmission speed (50 or 60 BPS), it takes 29 or 24 seconds (for 50 BPS) to establish communication with one terminal station.
If ten switches are installed for each distribution line and ten distribution lines for each bank, on the other hand, it is necessary for the master station to monitor the conditions of 100 switches under normal state. This means a considerable time required for monitoring.
Generally, the conditions of the switches of the distribution system do not undergo frequent changes, but only about once every one or several days. A change of the conditions of the switches of the terminal stations, however, should be desirably detected as early as possible for the purpose of early detection of the power failure or the like of the user. In order to constantly monitor the condition change that rarely occurs, therefore, a technique has been required to monitor any condition change for all the terminal stations repetitively in cycles of a short time.